GASEOUS NITRIDING OF BINARY Ni-Cr ALLOYS - OBSERVATION AND INTERPRETATION OF MICROSTRUCTURAL FEATURES

Gaseous nitriding of binary Ni-Cr solid-solution alloys was studied at 1125 degrees C over the range 1 to 6000 bar of N-2-pressure. At the specified temperature the nitriding response of the Ni-Cr alloys depends on the Cr-content in the initial alloy and activity (fugacity) of nitrogen at the gas/metal interface. Transition from cubic delta-CrN to hexagonal beta-Cr2N precipitation occurs within the reaction zone after nitrogenization at 1125 degrees C under nitrogen pressure 100-6000 bar when chromium content in the initial alloy is 28 at. % or higher. It was found that a ternary phase, pi (Cr12.8Ni7.2N4.0) is formed inside the Ni32Cr alloy upon cooling in nitrogen after nitriding at 1125 degrees C and 1 bar of N-2. Experimental evidence is presented that pi-phase is involved in peritectoid relations with beta-Cr2N and gamma-(Ni-Cr) solid solution. It was also demonstrated that nitriding behaviour of the Ni-Cr alloy can be rationalized using pertinent phase diagram information, but, in some cases, effect of mechanical stresses induced upon the internal precipitation can vitiate this prediction.